A volcano is a vent or fissure in Earth’s crust through which lava, ash, rocks, and gases erupt. An active volcano is a volcano that has erupted in the recent past. The mantle contains a weaker zone called asthenosphere. The material in the asthenosphere is called magma.
The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen, and argon.
Volcanoes are classified on the basis of nature of eruption and the form developed at the surface.
The Shield volcanoes are the largest of all the volcanoes on the earth, which are not steep. These volcanoes are mostly made up of basalt. They become explosive if in some way water gets into the vent, otherwise, they are characterized by low-explosive volcanoes. The forthcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone. For example, Hawaiian shield volcanoes
Composite Volcanoes are characterized by outbreaks of cooler and more viscous lava than basalt. They are constructed from numerous explosive eruptions. Large quantities of pyroclastic material and ashes find their way to the ground along with lava. This material gathers in the vicinity of the vent openings resulting to the creation of layers. Mayon Volcano in Philippines, Mount Fuji in Japan, and Mount Rainier in Washington are the major composite volcanoes in the world. The major composite volcano chains are Pacific Rim which known as the "Rim of Fire".
Calderas are known as the most explosive volcanoes of Earth. They are generally explosive in nature. When they erupt, they incline to collapse on themselves rather than constructing any structure. The collapsed depressions are known as calderas.
Flood Basalt Provinces
Flood Basalt Province volcanoes discharge highly fluid lava that flows for long distances. Many parts of the world are covered by thick basalt lava flows.
Mid-Ocean Ridge Volcanoes
These volcanoes are found in the oceanic areas. There is a structure of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central region of this ridge experiences recurrent eruptions.
The Volcanic eruptions result in the formation of landforms. The lava that is discharged during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on arriving on the surface or while the lava is still in the crustal portion.
According to the location of the cooling of the lava, igneous rocks are categorized as plutonic rocks and volcanic rocks. The lava that cools inside the crustal portions takes diverse forms. These forms are called intrusive forms.
Batholiths are the cooled portion of magma chambers. It is a large body of magmatic material that cools in the deeper depth of the crust moulds in the form of large domes. They appear on the surface only after the denudation processes eliminate the overlying materials. These are granitic bodies.
These are large dome-shaped intrusive bodies with a level base and linked by a pipe-like channel from below. It bears a similarity to the surface volcanic domes of the composite volcano, only these are located at deeper depths. It can be considered as the localized source of lava. The Karnataka plateau is patterned with dome hills of granite rocks.
When the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lapolith.
It is a wavy mass of intrusive rocks found at the base of synclines or at the top of anticline in folded igneous country. These wavy materials have a definite outlet to source beneath in the form of magma cavities.
The near horizontal bodies of the intrusive igneous rocks are called sill. The thick horizontal deposits are called sills whereas the thinner ones are called sheets.
Dykes are the most commonly found intrusive forms in the western Maharashtra area. When the lava makes its channel through cracks and the fissures it solidifies almost perpendicular to the ground. This gets cooled in the same position to grow a wall-like structure. Such structures are known as dykes. These are regarded as the feeders for the eruptions that led to the development of the Deccan traps.